This invention relates to a power supply unit comprising, between an input supply voltage terminal and a storage capacitor at an output terminal, an inrush current limiting circuit having a first semiconductor switch and a resistor in parallel to the switching path of the semiconductor switch. In parallel with the control path of the semiconductor switch is a parallel circuit comprising a first capacitor and a first resistor, as well as a switching path of a second semiconductor switch.
Power supply units for data transmission systems convert a DC voltage (supply voltage), which is derived by rectification from, for example, an exchange battery or the mains voltage, into a stabilized output voltage by means of a DC voltage converter. A storage capacitor is connected ahead of the DC voltage converter to bridge short-period supply voltage failures. The result of this is that, when the power supply unit is switched on, a high inrush current flows into the storage capacitor, which implies a dip in the supply voltage. In addition, the high inrush current may lead to an inadvertent triggering of safety cut-outs during the switch-on procedure.
DE-A 35 35 864 discloses a current limiting circuit which is designed to protect a load circuit comprising a storage capacitor and connected via this current limiting circuit in the case of short-period overvoltages and overcurrents. A field effect transistor is used for this purpose. The FET switching path is shunted by a parallel resistor and a current measuring resistor is connected in series therewith. To control the field effect transistor, a current evaluation circuit is connected to the current measuring resistor. The parallel resistor is connected to the current evaluation circuit in such a way that its current component is also evaluated. When the field effect transistor is cut off, the entire load current flows through the parallel resistor. The current evaluation circuit consists of a bipolar transistor whose emitter is connected to the current measuring resistor and whose base is connected to the source electrode of the field effect transistor via a resistor. If the voltage drop across the current measuring resistor exceeds a given maximum value, the bipolar transistor becomes conductive. This renders the control voltage (gate-source) of the field effect transistor zero and the field effect transistor is fully cut off. The entire current then flows through the parallel resistor. Below the maximum value of the voltage drop across the current measuring resistor the field effect transistor takes over a current component proportional to the voltage drop, while the remaining current component flows through the parallel resistor. In a further circuit modification, a delay unit consisting of a parallel circuit of a capacitor and a resistor is provided to delay the switching-on of the field effect transistor. A quick discharge of the storage capacitor must be ensured after a supply voltage failure in order to provide a short restoration time of the current limiting circuit. To this end, a PTC resistor is connected in parallel to the storage capacitor. The entire load current flows through the current measuring resistor, which may cause considerable power losses in the circuit.